1. Field of Invention
This invention relates to the field of systems for the storage and retrieval of encoded data on an optical recording medium. More particularly, this invention relates to the detection of and compensation for the recording error of pit edge extension associated with the physical alteration of the recording surface of an optical disk during the writing of data using pulses of laser light energy.
2. Prior Art
The recording of encoded data on optical media for subsequent retrieval is well known in the art. Generally, modulated light energy, such as a laser beam, is directed onto a light absorbing surface of an optical recording medium. The light energy is converted to thermal energy which physically alters the light absorbing surface region or an interior layer or layers in the medium to form an area of different reflectivity or different geometry, e.g. a pit, which is detectable optically during read operations. One of several modulation codes known in the art may be used to encode data, to control the generation of laser write pulses, and to control detection and retrieval of stored encoded data from the optical medium.
One disadvantage inherent in the use of laser light energy to store encoded data on an optical medium is that pitting cannot be completely restricted to the area defined by the period of laser beam irradiation during writing. When a pulse of laser light irradiates the medium surface during writing for a selected time as the medium turns, the leading and trailing edges of the pit extend somewhat beyond the laser on time because the thermal propagation of the laser energy on the recording surface continues for a time beyond the selected laser on-time. This pit extension depends on the thermal characteristics of the particular optical disk being used. The severity of pit extension is increased as the length of the laser irradiation is increased, due to the accumulation of thermal effects, resulting in distortion of the encoded data when read. This also results in an increased pit edge transition jitter.
Because of the problem of non-uniform pit extension and excessive pit edge transition jitter, optical systems in the prior art have read encoded data by detecting the presence or absence of a pit using the transition at the center of the pit, referred to as pulse position modulation (PPM), within selected clock time periods. A drawback of PPM is that it limits the maximum density of data that can be written to and read from the medium. That is, only one transition can be detected for each pit, whereas with pit edge transition detection, referred to as pulse width modulation, each pit has two transitions that represent data. Although pit center detection systems are capable of detecting pit edge transitions, according to the prior art, doing so would result in a high error rate due to the variation in spacing between edges caused by variation in pit edge extension and the resultant increased itter in the positioning of the pit edge transitions. Consequently, even though a pit edge transition detection scheme would have the great advantage of effectively doubling the data storage capacity of a medium, this technique is not generally used in the prior art. No known scheme is currently known in the art where pit edge detection is used in a write once optical recording system.
The only prior art optical storage systems wherein pit edge modulation is used is in CD-audio and CD-ROM optical recording systems. A typical storage capacity of greater than 550 megabytes on a 120 millimeter diameter CD-ROM disk is readily achievable due to the use of pit edge modulation and EFM coding. The success of this coding is enabled because of the ability to form high quality long pits with low edge itter on the medium. This is possible because the pits on a CD-ROM disk are generated on a photo-resist coated optical quality glass disk by a high precision and high power laser recording system under carefully controlled conditions. Such processing conditions are not duplicatable on conventional write-once or erasable optical recording systems.
Because the degree of pit extension is peculiar to each individual recording medium, another obstacle to the efficient use of a pit edge transition detection scheme is that a system designed to compensate for the pit extension characteristics of one medium must also be designed to compensate for the differing pit extension characteristics of any other medium to be used if interchangeability is to be achieved. Nothing in the prior art suggests how to achieve the advantages of a pit edge transition detection scheme without unacceptable error rates while also achieving media interchangeability.